A. Field of the Invention
The present invention relates to an extruding apparatus for extruding a synthetic resin and, more particularly, to an extruding apparatus for extruing a synthetic resin, comprising two extruders, a suply port of one of which is connected to a discharge port of the other.
B. Description of the Prior Art
A conventional extruding apparatus is disclosed in Japanese Patent Disclosure (Kokai) No. 48-79864. This extruding apparatus includes first and second extruders each having a cylinder which has a screw shaft therein. The second extruder, located at a front portion, has a single supply port connected to a discharge port of the first extruder, located at a rear portion. The supply port has a sectional area corresponding to the amount of a molding material supplied to the cylinder of the second extruder.
Accordingly, the molding material is softened by the first extruder, at the rear portion, into a semimolten state, and is supplied into a space between the cylinder and the screw shaft of the second extruder, through the supply port thereof. However, when the space becomes filled with semimolten resin, a problem occurs at the outer surface of a screw shaft except helical blades or flight of the screw shaft. More specifically, no axial feed effect occurs at the outer surface of the shaft, and the semimolten resin becomes attached to the outer surface of the shaft. The semimolten resin gradually accumulates on the already attached resin, and the sectional area of the space between the cylinder and the screw shaft becomes significantly reduced. In addition, the resin that has been attached for a long time is removed from the screw shaft and transferred to a discharge port. So that a discharge amount of the resin is unstabilized and quality of the extruded resin is degraded.
The second extruder disclosed in Japanese Patant Disclosure (Kokai) No. 48-79864 is provided with a vent hole for exhausting a gas generated when the molding material is melted. This conventional vent hole is situated adjacent to a position in a cylinder where pellets used as a molding material are heated and begin to melt, and passes through the cylinder perpendicular thereto. In addition, the vent hole is normally connected directly to a vacuum pump.
However, some of the resin melted near the vent hole is extruded outwardly therefrom, and is drawn by the vacuum pump thereby disturbing the operation of the pump. When the vacuum pump is unable to operate property, it must then be cleaned by temporarily stopping the operation of the extruder, or else the extruder must be driven by switching the vacuum pump to an auxiliary pump which is located in advance during cleaning of the pump. Such a process effectively prevents an extruding operation, and inevitably results in higher operating costs.